Polymorphic perversity: Crystal structures with many symmetry-independent molecules in the unit cell

An extensive analysis of polymorphic crystalline systems of organic compounds in which at least one member has a high number of molecules in the asymmetric unit (Z' > 2) has been carried out. Crystal structures are compared by traditional methods based on crystallographic cell reductions and powder patterns, as well as from stability considerations by comparing the distribution of molecule-molecule energies in the packing coordination sphere. The combination of these methods allows a safer detection of genuine polymorphism, since real space and reciprocal space information are seen to be complementary. In many cases, a clear-cut difference between polymorphic structures appears. However, there are also cases where X-ray structure determinations of "new" polymorphs can be better interpreted merely as low-quality redeterminations of previously found phases. The present study provides improved crystal structure recognition methods and clarifies some details of the molecular organization in crystal structures with high Z'. We discuss problems of how polymorphs are to be defined and make some suggestions about the conditions under which one polymorph or the other may be formed.